In oil or gas wells, well tools, such as hydraulic packers, are manipulated by fluid pressure in the well tubing. To obtain increased pressures in the well tubing, a pump out sub is used that plugs the well tubing so that pressure may be increased to actuate the well tool. The pump out sub contains a seat that receives a plug, such as a ball or dart, which is dropped down the well tubing. After actuation of the well tool, the plug is dislodged from the seat by increasing fluid pressure to a level that a shear pin holding the seat in place is sheared. The seat moves downward within the well tubing and the plug is disassociated therefrom passing downward through and out of the well tubing. An example of a pump out sub is described in U.S. Pat. No. 4,510,994, issued Apr. 16, 1985, which is incorporated herein by reference.
Conventional pump out subs are susceptible to formation surge because of the differential pressures that must be used to release the plug. Accordingly, there is a need for an improved pump out sub that reduces or eliminates formation surge.